Napa County Groundwater Sustainability Planning

Like much of California’s Coast Range, Napa County’s complex geologic history exerts a strong influence on present day water resources management. Throughout the twentieth century, productive agricultural lands, population growth, and limited surface water supplies have had the result of increasing reliance on groundwater.

We advanced the hydrogeologic conceptualization of the Napa Valley by synthesizing over a dozen historic geologic maps and studies, interpreting over 1,300 Well Completion Reports, and developing new geologic maps consistent with modern plate tectonics theory. We also conducted an extensive analysis of stream flow and base flow conditions to quantify the contributions of groundwater to instream flows. These efforts resulted in an improved understanding of the character and extent of aquifer materials and the influence of groundwater conditions on stream flow. Based on those findings and the results of other analyses performed concurrently, LSCE worked closely with the County to update procedures used to evaluate and permit proposed uses of groundwater. The revised County procedures reflect the current understanding of groundwater availability, mutual well interference potential, and stream flow depletion potential.

In response to the Sustainable Groundwater Management Act, we additionally prepared a Basin Analysis Report (BAR) for the Napa Valley Subbasin, consistent with the requirements for Groundwater Sustainability Plan Alternatives. The BAR included a new 28-year water budget analysis and water balance mathematical model, as well as an updated analysis of sustainable yield. The BAR was submitted to the California Department of Water Resources in December 2016, five years ahead of the deadline for Groundwater Sustainability Plan submittal.

Significant Project Elements

• Updated hydrogeologic conceptualization for Napa Valley including eight new geologic cross-sections and new geologic mapping of subsurface stratigraphy informed by a review of over 1,300 well completion reports
• Multi-year technical services support to facilitate the work of the Napa County Groundwater Resources Advisory Committee
• Monitoring network evaluation and monitoring plan development
• Water budget analysis and evaluation of SGMA sustainability criteria
• Updated groundwater permit procedures that reflect best-available information and analysis of the potential for mutual well interference and streamflow depletion